The present invention is directed to direction-finding antenna systems. It is directed particularly to systems employing circular arrays of antenna elements in which signals from several sources are to be monitored simultaneously.
A powerful device that has been proposed for use in systems for electromagnetic surveillance is the two-dimensional compressive receiver. The two-dimensional compressive receiver performs a two-dimensional Fourier transformation in time and one spatial dimension. Typically, the outputs of a linear array of antenna elements--i.e., of an array in which the elements are disposed on a line--are applied to the input ports of the two-dimensional compressive receiver, and the two-dimensional compressive receiver generates signals at a plurality of output ports. The compressive receiver generates a complete set of outputs within each sweep of the compressive receiver's chirped local oscillator period. Signals of a single frequency are compressed in time, and the time during a sweep at which a time-compressed output occurs indicates the temporal frequency of the antenna signal that gave rise to it. The output port from which the signal issues indicates the spatial-frequency component at the antenna caused by the signals from that source.
Spatial frequency is the rate of phase progression of signals from a given source with respect to position along the antenna array; if the direction of the source forms a large angle with the normal to a linear array and the temporal frequency of the signal is high, then its spatial frequency will be high. On the other hand, small angles and low temporal frequencies result in low spatial frequencies. The angle of the source is indicated by the ratio of the spatial frequency of a component to its temporal frequency.
It is thus apparent that the two-dimensional compressive receiver can identify the temporal frequencies and directions of many signal sources simultaneously.
This simple relationship between direction, spatial frequency and temporal frequency only is obtained, however, if the antenna array is linear, so the two-dimensional compressive receiver indicates the direction of the source readily only if the array is linear. However, it is sometimes desirable to employ circular arrays of antenna elements, and it would be beneficial to obtain the powerful processing abilities of the two-dimensional compressive receiver in a system employing a circular array. To do so is an object of the present invention.
It is a further object of the present invention to provide the direction information in a manner that is independent of temporal frequency so that the direction indication can be generated not only by two-dimensional transform devices but also by devices for performing one-dimensional spatial Fourier transformations.